1. Field of the Invention
The present invention relates to slurry dispensers that dispense slurry to a chemical-mechanical polisher used in semiconductor fabrication and, more particularly, to a slurry dispenser that outputs a filtered slurry to a polisher at a constant flow rate over the lifetime of the filter.
2. Description of the Related Art
A chemical-mechanical polisher is a device that removes excess material from the top surface of a semiconductor wafer. Chemical-mechanical polishers are commonly used to planarize the topography of a wafer, and to form damascene structures that are embedded in an insulation layer on a wafer.
FIG. 1 shows a block diagram that illustrates a conventional chemical-mechanical polisher 100. As shown in FIG. 1, polisher 100 includes a round polish platen 110, a round pad 112 that is connected to platen 110, and a slurry dispenser 114 that dispenses a slurry 116 onto pad 112. Slurry 116 includes water, a number of chemicals, and an abrasive material that has a large number of particles.
In addition, polisher 100 also includes a wafer carrier 120 that holds a wafer 122 so that the top surface of wafer 122 is parallel to the top surface of pad 112. Polisher 100 further includes a vertical carrier 124 that moves wafer carrier 120 up and down so that the semiconductor materials formed on the top surface of wafer 122 are brought into contact with pad 112.
In operation, pad 112 is rotated via platen 110 at a high rate of speed, slurry 116 is dispensed to pad 112, and wafer 122 is rotated via carrier 120 at a high rate of speed and lowered until wafer 122 makes contact with pad 112. Pad 112 and slurry 116 then remove the materials formed on the top surface of wafer 122, beginning with the peaks, for as long as pad 112 and wafer 122 remain in contact.
One of the problems with chemical-mechanical polisher 100 is that when oversized particles are present in the abrasive material in slurry 116, the surface of wafer 122 can become scratched and may affect the polishing removal rate and non-uniformity. With very small line widths, these scratches and degraded process characteristics can destroy or degrade the devices being fabricated on wafer 122.
One approach to preventing scratches from oversized particles is to add a filter to slurry dispenser 114 that removes the oversized particles from slurry 116. One problem with filters, however, is that filters increasingly restrict the flow of slurry 116 over time as the filters catch more and more oversized material. Eventually, the filters clog up and the flow of slurry stops.
Changes in the slurry flow rate effect the removal rate of the wafer material that is in contact with the pad which, in turn, makes it difficult to calculate how long the wafer material should remain in contact with the pad. In addition, polishers typically require a minimum slurry flow rate to remove material from a wafer, and prevent damage to the wafer.
As a result, to avoid damaging the wafer, the filter must be replaced before the decreasing slurry flow rate drops below the minimum slurry flow rate. Thus, there is a need for a slurry dispenser that outputs a filtered slurry, indicates when the filter needs to be replaced, and outputs the filtered slurry at a constant flow rate throughout the lifetime of the filter.
The present invention provides a slurry dispenser that utilizes a filter to remove oversized particles from a slurry to reduce the effects of scratches. In addition, the slurry dispenser of the present invention indicates when the filter needs to be replaced, and maintains a constant flow of slurry through the filter throughout the lifetime of the filter.
A slurry dispenser in accordance with the present invention includes a pump that receives a slurry from a slurry supply, and outputs a pumped slurry. The pump outputs the pumped slurry with a flow rate at a pump speed. The pump speed is controlled by a pump speed control signal.
The slurry dispenser also includes a slurry filter that removes oversized particles from the pumped slurry. The slurry filter has an input and an output. The dispenser additionally includes a flow meter that measures a flow rate of the pumped slurry, and outputs a measured flow signal that indicates a measured flow rate.
Further, the slurry dispenser includes a flow controller that receives the measured flow signal, compares the measured flow rate to a reference set point rate, and controls a value of the pump speed control signal in response to the difference between the measured flow rate and the reference set point rate.
The present invention also includes a method of dispensing slurry onto a pad of a chemical-mechanical polisher that includes the step of pumping a slurry from a slurry supply to output a pumped slurry. The pumped slurry has a flow rate at a pump speed. The pump speed is controlled by a pump speed control signal.
The method also includes the step of filtering the pumped slurry with a filter to remove oversized particles from the pumped slurry. The method additionally includes the steps of measuring a flow rate of the pumped slurry, and outputting a measured flow signal that indicates a measured flow rate.
The method further includes the steps of receiving the measured flow signal, comparing the measured flow rate to a reference set point rate, and controlling a value of the pump speed control signal in response to the difference between the measured flow rate and the reference set point rate.